We're working out what we think - we'll add our view here!
Sierra Designs says:
The Flex Lumbar collection takes the same expanding gusset found in our Flex Capacitor, and shifts it vertically to work as a lumbar pack. Integrated water bottle pockets and enough room to stash a layer and some food means you have your bases covered for any day hike. Flex it open when you need to carry more, or cinch it down when you want a lower profile pack.